The present invention relates to textured rolls and roll texturing processes and related rolling process enhancements for both cold and hot rolling of aluminum and its alloys, such rolling process enhancements resulting when the textured rolls are used to reduce the thickness of the rolled material. The invention also relates to enhancements to the functional properties of the strip surface (in the case of cold rolling) and/or the slab surface (in the case of hot rolling) that result when rolling with textured rolls. The roll texture permits massive reductions (the word "massive" referring to cold rolling processes involving product thickness reductions at or in excess of 55% ) in the thickness of strip material passing through a stand of a cold rolling mill due to the ability of the texture to entrap and thereby draw adequate quantities of lubricant into the roll bite, while simultaneously forming a substantially non-directional strip surface texture that results from micro-backwards extrusion of microscopic quantities of strip material into each element of the roll texture, such small quantities being subject to concurrent smearing due to the kinematics associated with massive thickness reduction rolling processes. The present invention also relates to the application of the aforementioned cold rolling roll textures to hot rolling processes wherein a slab at elevated temperatures is reduced in thickness with rolls textured with the method of the invention during normal thickness reduction hot rolling (the word "normal" pertaining to approximately 40% thickness reduction rolling). The roll textures act to entrap lubricant in what is otherwise a lubricant-depleted roll bite (the depletion of lubricant resulting from the extreme temperatures associated with hot rolling), such lubricant then being expelled to the roll/slab interface upon which time it acts to substantially minimize material transference due to adhesion between the roll surfaces and the slab surfaces and minimizes rolled-in debris and smudge on the slab surface as it enters the cold rolling stands. Note that the word "strip" will be used throughout the following discussion to refer to cold-rolled sheet products having a nominal thickness that is less than 0.125 inch. The word "slab" will be used throughout the following discussion to refer to hot-rolled plate products having a nominal thickness that is equal to or greater than 0.125 inch. The words "workpiece" and "product" are generic references to either a strip or slab and hence are used with reference to both the cold and hot rolling processes considered herein.
Strip thickness reductions in a single pass through a cold mill stand involving "hard" aluminum alloys, i.e., alloys other than 1100, are usually limited to about 45%. Massive thickness reductions (55% or more) with ground work rolls lead to lubricant film breakdown and the undesirable herringbone effect, which is a period marking of a strip surface at some angle relative to the rolling direction. The herringbone defect is a visual manifestation on the strip surface of a stick-slip phenomenon in the roll bite, which is caused by a periodic changeover between hydrodynamic, thick film lubrication to boundary film lubrication, the latter being essentially no lubrication at various locations in the roll bite and involving metal-to-metal contact. Aluminum alloys are particularly susceptible to the herringbone phenomenon, as discussed in Chapter Six of Tribology in Metalworking. Friction Lubrication and Wear by J. A. Shey, American Society for Metals, 1983. A more familiar manifestation of a stick-slip phenomenon, for example, occurs when automobile windshield wipers begin to wear out. This results in a loud vibrating sound and an imperfect wiping action along the windshield.
Further, rolling speeds are typically lower when thickness reductions are higher, and this leads to thinner lubricant films in the roll bite. In addition, ground finishes on work roll surfaces tend to promote lubricant channeling out of the roll bite instead of retention of lubricant in the roll bite in both hot and cold rolling processes. Longitudinal roll grind marks on an aluminum strip surface, which result from the imprint of a ground finish on a work roll with an intermittent texture, are visible in FIG. 1 of the drawings.
Other process problems associated with massive thickness reduction cold rolling processes are excessive heat generation due to the work of plastic deformation of the strip, excessive slip in the backup roll drives and excessive negative forward slip between the strip and the work rolls. Forward slip is defined as the difference between the strip surface speed and the roll surface speed (both quantities being measured in the exit plane of a roll bite), this difference then being divided by the strip surface speed in the exit plane. Negative forward slip indicates a possible condition of mill instability where the work roll surface speed exceeds the strip speed as the strip exits the roll bite and hence the roll surface skids over the strip surfaces either periodically or in a constant fashion.
Ground roll surfaces also promote degradation of the cold rolled strip surface brightness through the proliferation of micro-cracks or fissures that form transversely to the rolling direction. The generation of transverse fissures during rolling of a strip of material with work rolls having ground finishes and the resulting degradation in the functional properties of the rolled strip have been discussed in detail in the Applicants' U.S. Pat. No. 4,996,113 entitled "Brightness Enhancement with Textured Roll" issued Feb. 26, 1991.
With the above problems in mind, it should be noted that cold rolled strip and hot rolled slab products are the most widely used materials in the world. It is, therefore, no surprise that rolling is the most important bulk metal forming process from an economic standpoint. In the steel and aluminum industries alone, millions of tons of metal are processed each year by hot and cold rolling, the rolling process beginning with cast ingots. Rolled materials are used in the manufacture of a large variety of products which include such modern-day necessities as automobiles, household appliances, aircraft structural components and beverage cans.
Tribological conditions in the bite of the work rolls and between the work rolls and the backup rolls have a significant impact on rolling mill performance and the quality and ultimate marketability of the rolled products. (The term "tribology" refers to the physical conditions and processes existing between contacting and rubbing surfaces). Optimum tribological conditions for rolling result, in part, through a proper combination of lubricant chemistry, coolant sprays, roll surface morphology, rolling speeds, product entry gauges, strip product thickness reduction ratios and the thermomechanical properties of the material being reduced in thickness. For example, an adequate lubricant is one that prevents substantial adhesive transfer between the workpiece and the rolls as well as inhibits retransfer of adhered workpiece material from the roll surface back to the workpiece surface, and simultaneously acts as a roll stack coolant. Improper roll surface preparation, coupled with an inadequate lubricant medium, leads to such tenacious problems as roll surface pickup, which is the transference or adherence of workpiece surface material to the roll surface, followed by retransfer of adhered workpiece material to the surface of the rolled product (FIGS. 4a to c and FIG. 5), wear debris and smudge generation in cold and hot rolling, and transverse fissure formation in a cold-rolled strip surface and broken areas in the surface of a hot-rolled slab. When a work roll becomes coated with transferred workpiece material during rolling, the roll surface no longer meets the initial surface roughness specification to which it was initially finished. Rather, the roll coating of adhered workpiece material is typically quite rugged and acts to abrade the workpiece surface leading to greater debris generation. With reference to aluminum rolling processes, wear debris consists of aluminum fines, oxides, and other solid debris. Smudge refers to a mixture of carbonaceous oil residue plus wear debris particles that are in general smaller than material filtered out of the lubricant re-circulation system employed in both cold or hot rolling mills. Smudge may consist of various chemical by-products from reactions within the roll bite between the lubricant and the workpiece surface material (examples of these by-products are metal soaps and lubricant polymers). Such by-products cause the surface of the rolled product to appear to be very dirty and hence render the product undesirable to the customer of the product. Metal pickup on a work roll with subsequent retransfer of adhered material to the workpiece surface is detrimental from both a process and product standpoint. Excessive levels of pickup and smudge generation can lead to large product losses and production downtime since the roll surfaces will tend to need more redressing (e.g., grinding). Transverse fissures that form in a strip surface substantially degrade strip appearance and often make the product less appealing to a potential customer. For example, transverse fissures in the exterior surface of an aluminum beverage can will degrade the image clarity of the can label, as discussed in the Applicants' U.S. Pat. No. 5,250,364 entitled "Rolled Product with Textured Surface for Improved Lubrication, Formability and Brightness" issued Oct. 5, 1993. At elevated rolling speeds and heavy thickness reductions during cold rolling, heat streaks, the herringbone phenomenon and center or edge buckling are common problems that decrease productivity and prohibit acceptable quality of the rolled product.
Cold rolling is typically conducted in the boundary and mixed film lubrication regimes, as discussed in Applicants' U.S. Pat. No. 4,996,113 noted above. In the mixed lubrication regime, the ratio of lubricant film thickness to composite surface roughness is about one. In this regime, lubricant flows between channels surrounding asperity contacts between the strip and work roll surfaces. Pan of the compressive load within the roll gap is carried by the asperities (i.e. high points in the rugged irregularities that comprise the surface microgeometry relative to a measuring reference), and part of the load is carried by pressurized lubricant pockets between the asperities. Traction between the two surfaces is moderate and, hence, wear of the surfaces is high, resulting in a large amount of wear debris particles being generated in the roll/strip interface through micro-abrasion between the work roll and strip surfaces. When the ratio of lubricant film thickness to composite roughness is substantially less than one, a boundary lubrication regime is prevalent. When boundary films that coat the strip and work roll surface roughness break down, metal transfer is more likely since there is no intermediate medium through which separation of the surfaces is achieved, and the wear rate is high, resulting in surface damage to the work rolls, the backup rolls and the rolled product.
Further, roll separation force and roll torque are strongly influenced by the tribological conditions prevailing in the roll bite. High roll bite friction affects strip shape, flatness and product surface quality, and may result in non-homogeneous deformation of the workpiece strip, with an increased incidence of unwanted residual stress in the strip. High friction in the roll bite also limits the ability of the mill to take massive strip thickness reductions or to roll wide or thick metal. The need to roll wider strip is market driven by such industries as the beverage can industry; obviously, if one can roll wider sheet, then a larger number of beverage container blanks can be stamped per sheet width. An example of a rolling mill design that is purported to allow the rolling of wider sheet is found in U.S. Pat. No. 4,173,133 to Ima et al.
The surface aesthetic and functional properties of rolled products tend to be solely determined by the tribological conditions in the roll bite. This is true in both hot and cold rolling operations. Hot line pickup, heat streaks, smudge, rolled-in dirt, brightness and distinctness of image of the workpiece surface are just a few of the more common surface aesthetic issues. Performance of a strip material in a secondary forming operation (such as stamping, drawing, and ironing), the prevalence of a substantially isotropic frictional force component in secondary metal forming processes, such as the pressing of autobody parts, and cleanliness of the strip and the extent to which buildup of smudge on metal forming tools occurs, are examples of functional properties of rolled products that are influenced by rolling tribology.
Roll grinding has been the principal means of roll surface preparation since the nineteenth century. Roll surfaces are ground in a number of grinding operations to a specified arithmetic means roughness (R.sub.a) prior to installation in a rolling mill. The grinding operation imparts a directional pattern on the roll surface, which finish, in turn, imparts a directional finish onto the strip surface during rolling, as discussed earlier in regard to FIG. 1. Unfortunately, a ground roll finish carries the previously mentioned list of problems into the rolling process and can be especially detrimental to strip surface quality. Even though these and other problems continue to burden aluminum sheet and plate manufacturers, few alternatives to a ground roll surface have been explored in the past. The reluctance to consider alternative roll surface morphologies in the aluminum industry may be due, in part, to the general failure of shot-blasted work roll surfaces in heavy thickness reduction aluminum rolling processes, even though shot blasting is successfully employed throughout the steel industry. One of the reasons shot blasting has been used in the steel industry is that steel reductions are typically not substantial or massive, as is the case with aluminum. Also, hard steel surfaces are not prone to damage as much as softer aluminum surfaces. In order to effect a compromise with the negative aspects of rolling with ground rolls, the aluminum industry has heavily invested in lubricant development, sophisticated rolling mill controls, improved grinding practices, etc., but no significant investment has been made in alternative work roll surface preparation technologies.
British specification No. 1,486,321, published in 1977, discusses increasing strip thickness reduction during cold rolling by the use of a roll texture consisting of bowl-shaped depressions created by sand blasting the rolls to a maximum bowl depth in the range of 1.0 to 15 microns. The rolls were then ground or polished following the sand blasting in order to prevent "possible adhesion of the workpiece with a roll not yet run-in" (column 3, lines 41 and 42). The bowls impeded the escape of lubricant from the roll bite entry zone, and the product finish was substantially improved. It is difficult to engineer roll surfaces with sand blasting, as the sand particles strike the roll surface in a random fashion, and hence, a surface finish comprised of discrete elements having similar dimensions is not possible. The expulsion of lubricant entrapped by a sand blasted work roll during an aluminum rolling process is substantially nonuniform and can lead to regions of the roll/workpiece interface that are depleted of lubricant. In addition, sharp cutting edges in the roll surface will abrade aluminum strip or slab material leading to excessive wear debris generation. Hence, the method of this disclosure cannot be used to effect either normal thickness reduction hot rolling or massive thickness reduction during cold rolling of aluminum and its alloys.
In the late 1970's, focused energy beam roll texturing technologies were developed to economically produce surface morphologies that can be "engineered" to influence rolling process and product performance. For example, the more familiar annular crater textures that are now quite common in the European and Japanese steel industries are typically used to emboss strip surfaces during light thickness reduction rolling, i.e., during so-called "temper rolling operations". The strip surface then becomes self-lubricating during secondary forming processes. FIG. 1 in the present application shows an aluminum surface embossed with a roll crater texture using a light thickness reduction on the order of 1 to 3%. The strip surface shows the imprint of the roll surface which was imparted to the strip surface in the absence of any significant strip surface smearing. There is a substantial amount of literature available on the annular crater work roll surface texture and the effects it has on strip performance. This includes Applicants' U.S. Pat. No. 5,025,547, the August 1991 issue of Iron and Steel Engineer and the Applicants' Focused Energy Beam Work Roll Surface Texturing Science and Technology in the July 1993 issue of the Journal of Materials Processing and Manufacturing Science.
Generally, work roll surface crater textures developed over the past ten years are not meant for either normal thickness reduction hot rolling processes that involve rolling speeds in the 400 ft/min-1500 ft/min range, or massive thickness reduction cold rolling processes that involve high rolling speeds (e.g., on the order of 4,000 ft/min), as the crater rims can (a) promote backup roll surface damage since very large loads are transferred between the work roll and backup roll surfaces over the microscopic crater rim areas; (b) generate unacceptable levels of wear debris during aluminum rolling processes to the point where the oil filtration house for the mill becomes clogged thereby necessitating that the mill be shut down; (c) increase the load on mill stands; (d) wear down to the point where they no longer have a significant effect on the rolling process or the product surface; (e) produce undesirable aesthetic effects on aluminum strip and slab surfaces. As an example of the latter situation, FIGS. 2a and 2b in the present application show aluminum strip surfaces rolled at 35% and 60%, respectively, with the asymmetric hump work roll surface texture disclosed in U.S. Pat. No. 4,806,731 to Bragard et al. and shown in FIG. 3 of the present drawings. The roll texture is not faithfully embossed onto the strip surface, as the strip surface takes on a substantially directional component. The work roll surface texture of the Bragard et al. roll was originally designed for embossing steel strip during light thickness reduction rolling processes. The Applicants' pending application Ser. No. 238,249 entitled "Sheet Product Produced by Massive Reduction in Last Stand of Cold Rolling Process" contains a method by which massive thickness reductions can be effected in the last stand of cold rolling with an hemispherical bowl texture on the work roll surfaces. This method is limited to cold rolling since seizure of strip surface material results during normal thickness reduction hot rolling. Seizure of microscopic quantities of an aluminum strip material into hemispherical bowls on a work roll surface due to hot rolling is shown in FIG. 5. In addition, the method of the Applicants' pending application produces a strip surface texture in the last stand of cold rolling that totally differs in shape and functional performance than the strip texture produced with the method of the present invention.